I. Field of the Invention
The present invention relates to coding. More particularly, the present invention relates to a novel and improved technique for performing maximum a posteriori probability (MAP) decoding.
II. Description of the Related Art
xe2x80x9cTurbo codingxe2x80x9d represents an important advancement in the area of forward error correction (FEC). There are many variants of turbo coding, but most use multiple encoding steps separated by interleaving steps combined with iterative decoding. This combination provides previously unavailable performance with respect to noise tolerance in a communications system. That is, turbo coding allows communications at Eb/N0 levels that were previously unacceptable using the existing forward error correction techniques.
Many systems use forward error correction techniques, and therefore would benefit from the use of turbo coding. For example, turbo codes could improve the performance of wireless satellite links where the limited downlink transmit power of the satellite necessitates receiver systems that can operate at low Eb/N0 levels. Using turbo codes in a wireless satellite link could reduce the dish size for a digital video broadcast (DVB) system, or alternatively, allow more data to be transmitted.
Digital wireless telecommunication systems such as digital cellular and PCS telephone systems also use forward error correction. For example, the IS-95 over-the-air interface standard, and its derivatives, such as IS-95B, define a digital wireless communications system that uses convolutional encoding to provide coding gain to increase the capacity of the system. A system and method for processing RF signals substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone Systemxe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference (""459 patent).
Many types of turbo coding require more complex circuitry than the previous available FEC coding techniques. For example, many turbo codes use maximum a posteriori probability (MAP) decoders, which are more complex than conventional Viterbi trellis decoders. For high data rate applications, the more complex MAP decoder can reduce the data rate at which data transmission can be performed. Thus, it would be highly desirable to have a MAP decoder that could operate at high data rates.
The present invention is a novel and improved technique for performing maximum a posteriori probability (MAP) decoding, with particular application to turbo, or iterative, coding techniques. In accordance with one embodiment of the invention, a set MAP decoders are applied in parallel to different sections of a frame symbol estimates to be decoded. Each MAP decoder generates decisions for the section of symbol estimates received. In one embodiment, initialization values are generated. The MAP decoders use the initialization values to start decoding at each section where the section begin at the middle of the frame.